Surveying the Ōhau Canal
The Waitaki Hydroelectric Scheme in the Tekapo Basin and Waitaki Valley is NZ’s largest hydroelectric power scheme. The picturesque Lakes Tekapo and Pukaki hold around 50% of the country’s hydroelectric storage, and the scheme provides around 16% of our annual electricity needs.
NZ’s largest electricity generator, Meridian Energy Ltd owns the portion of the scheme below Lake Pukaki. The chilly glacial water is conveyed from the lakes to the various power stations by a network of manmade canals constructed in the second half of the 20th century.
To make the canals watertight they are lined with impervious material which is then protected by a layer of rocks to prevent erosion from the fast-moving current.
A Surveying Challenge in a Critical Waterway
Meridian wanted to check for the possibility of erosion or damage to the liner of the Ōhau Canal, between Lake Ōhau and the Pukaki Canal.
Investigating this was challenging for a number of reasons. The canal needs to be kept full at all times to maintain electricity generation.
The liner has traditionally been inspected by divers however, they are not able to give accurate information on the extent or exact location of any damage.
Using Unmanned Survey Vessels for a Smarter Solution
It isn’t practical to employ manned boats in this canal due to the various obstructions such as bridges and salmon rafts, not to mention the lack of any boat ramps. So, Meridian turned to Fox & Associates with our expertise in surveying unmanned surface vessels (USVs – radio-controlled boats).
These boats are small enough to be manually lifted into the canals and to navigate the various obstructions and are fitted with survey-grade GPS and sonar equipment for accurate mapping of the canal bed.
Two sections of the canal totalling 2.4km were identified for surveying. As with all surveys there were challenging safety aspects to be overcome. In this case the work was to be undertaken in the depths of winter, so it was critical that staff kept warm and dry.
Meridian was also concerned lest the USV lose power and be drawn into the generating equipment, so the boats needed to be kept on a tether at all times. Both of our USVs were employed, the lower-powered but more stable BathyCat being more suitable in times of low water flow while the new Jettec jet boat was better able to handle faster flows.
As well as the USV survey, a UAV (unmanned aerial vehicle or drone) survey was conducted to obtain LiDAR data and aerial imagery for the portion of the canal above and immediately below the waterline.
The latter being limited by the glacial ‘flour’ that gives the lakes their distinctive turquoise colour but which also severely limits visibility.
The various datasets were merged to form a single 3d model of the canal.
Clear Data, Confident Decisions
With the data acquired the next challenge was to compare the current canal bed with the original ‘as-built’ state. No digital as built was available but design cross-sections were provided, and these were able to be lined up with the measured model. The positions of the salmon farm rafts were also plotted so that Meridian can study potential impacts these may have caused.
Our 3d data-wrangling experts were able to compare the surveyed model against the original design and create drawings showing the differences that represent erosion or accumulation of material. Meridian can use this information to investigate any areas of concern and to manage the canals including the effect of the salmon rafts.
This project typifies the Fox approach of looking for a better way to solve a client’s problem using our hard won expertise and cost-effective technology that is cost effective. Meridian was very impressed with the outputs and the level of useful information it provided to the decision makers. The team can’t wait for their next canal survey – maybe in summer next time though!